On viewing the lush plant growth of a tropical rain forest, most people would conclude that the soil beneath it is rich in nutrients. However, although rain forest soils are highly variable, they have in common the fact that abundant rainfall washes mineral nutrients out of them and into streams. This process is known as leaching. Because of rain leaching, most tropical rain forest soils have low to very low mineral nutrient content, in dramatic contrast to mineral-rich grassland soils. Tropical forest soils also often contain particular types of clays that, unlike the mineral-binding clays of temperate forest soils, do not bind mineral ions well. Aluminum is the dominant cation (positively charged ion) present in tropical soils; but plants do not require this element, and it is moderately toxic to a wide range of plants. Aluminum also reduces the availability of phosphorus, an element in high demand by plants.

High moisture and temperatures speed the growth of soil microbes that decompose organic compounds, so tropical soils typically contain far lower amounts of organic materials (humus) than do other forest or grassland soils. Because organic compounds help loosen compact clay soils, hold water, and bind mineral nutrients, the relative lack of organic materials in tropical soils is deleterious to plants. Plant roots cannot penetrate far into hard clay soils, and during dry periods, the soil cannot hold enough water to supply plant needs. Because the concentration of dark-colored organic materials is low in tropical soils, they are often colored red or yellow by the presence of iron, aluminum: and manganese oxides; when dry, these soils become rock hard. The famous Cambodian temples of Angkor Wat, which have survived for many centuries, were constructed from blocks of such hard rain forest soils.

Given such poor soils, how can lush tropical forests exist? The answer is that the forest's minerals are held in its living biomass—the trees and other plants and the animals. In contrast to grasslands, where a large proportion of plant biomass is produced underground, that of tropical forests is nearly all aboveground. Dead leaves, branches, and other plant parts, as well as the wastes and bodies of rain forest animals, barely reach the forest floor before they are rapidly decayed by abundant decomposers—bacterial and fungal. Minerals released by decay are quickly absorbed by multitudinous shallow, fine tree feeder roots and stored in plant tissues. Many tropical rain forest plants (like those in other forests) have mycorrhizal (fungus-root) partners whose delicate hyphae spread through great volumes of soil, from which they release and absorb minerals and ferry them back to the host plant in exchange for needed organic compounds. The fungal hyphae are able to absorb phosphorus that plant roots could not themselves obtain from the very dilute soil solutions, and fungal hyphae can transfer mineral nutrients from one forest plant to another. Consequently, tropical rain forests typically have what are known as closed nutrient systems, in which minerals are handed off from one organism to another with little leaking through to the soil. When mineral nutrients do not spend much time in the soil, they cannot be leached into streams. Closed nutrient systems have evolved in response to the leaching effects of heavy tropical rainfall. Evidence for this conclusion is that nutrient systems are more open in the richest tropical soils and tightest in the poorest soils.

The growth of organisms is dependent on the availability of nutrients, none of which is more important than nitrogen. Although there is an abundant supply of nitrogen in Earth’s atmosphere, it cannot be absorbed by plants unless it is “fixed,” or combined chemically with other elements to form nitrogen compounds. Nitrogen-fixing bacteria help tropical rain forest plants cope with the poor soils there by supplying them with needed nitrogen. Many species of tropical rain forest trees belong to the legume family, which is known for associations of nitrogen-fixing bacteria within root nodules. Also, cycads (a type of tropical plant that resembles a palm tree) produce special aboveground roots that harbor nitrogen-fixing cyanobacteria. By growing above the ground, the roots are exposed to sunlight, which the cyanobacteria require for growth. Nitrogen fixation by free-living bacteria in tropical soils is also beneficial.

在观赏热带雨林郁郁葱葱的植物生长时，大多数人会认为它下面的土壤营养丰富。然而，虽然雨林土壤成分多变，但它们的共同之处在于丰富的降雨将矿物质营养物质从其中冲刷出并流入溪流。这个过程被称为浸出。由于雨水浸出，大多数热带雨林土壤的矿物质含量都很低，与富含矿物质的草地土壤形成鲜明对比。热带森林土壤通常也含有特殊的粘土，与温带森林土壤的矿物结合粘土不同，它们不能很好地结合矿物离子。铝是热带土壤中主要的阳离子（带正电的离子）， 但植物不需要这种元素，并且对很多植物都有中等毒性。铝也降低了磷的可用性，但磷是植物极其需要的元素。 高湿度和高温加速了分解有机化合物的土壤微生物的生长，因此热带土壤的有机物质（腐殖质）的含量通常比其他森林或草地土壤低得多。由于有机化合物有助于松散粘土土壤，保持水分，结合矿物质营养，热带土壤中有机物质的相对缺乏对植物是有害的。植物的根部不能穿透到坚硬的粘土土壤中，在干旱的时候，土壤不能容纳足够的水来满足植物的需要。由于热带土壤中深色有机物质的浓度较低，因此在铁、铝和锰氧化物的存在下，它们通常呈现红色或黄色; 干燥时，这些土壤变硬。著名的柬埔寨庙宇吴哥窟已经屹立了好几个世纪，它就是用这种坚硬的雨林土块砌成的。 鉴于土壤如此贫瘠，那郁郁葱葱的热带森林是如何存在的？答案是，森林的矿物质被保存在其生物的体内——树木、其他植物和动物身体中。与地下生产大量植物生物的草地相比，热带森林几乎全部在地上。死叶、树枝等植物部分，以及热带雨林动物的废物和尸体，几乎还没有到达森林的地面，就被大量的细菌和真菌腐生物迅速分解。腐烂释放出来的矿物很快被众多浅而细的植物根系吸收并储存在植物组织中。许多热带雨林植物（像其他森林中的那些植物一样）具有菌根（真菌根）伴侣，其精细菌丝通过大量土壤扩散，从中释放和吸收矿物质并将它们运送回宿主植物以换取所需的有机物化合物。真菌菌丝能够吸收植物根本不能从极稀土壤溶液中获得的磷，而且可以将矿物营养物从一种森林植物转移到另一种。因此，热带雨林通常具有所谓的封闭营养系统，其中矿物质从一个生物体转移到另一个生物体，几乎没有渗漏到土壤中。当矿物质营养素在土壤中消耗的时间不多时，它们不会被浸出进入溪流。封闭的营养系统发展，以应对热带降雨的严重浸出效应。这一结论的证据是，营养系统在热带土壤最丰富的地区更为开放，在最贫瘠的土壤中更为紧密。 有机体的生长取决于营养素的可获得性，其中没有一种比氮更重要。虽然地球大气中氮气供应充足，但除非固定，否则不能被植物吸收，或与其他元素化学结合形成含氮化合物。固氮菌帮助热带雨林植物通过向它们提供所需的氮来应对那里的贫瘠土壤。许多热带雨林树种属于豆科植物，这种植物以根瘤中固氮细菌的联合而闻名。而且，铁树（一种类似于棕榈树的热带植物）长出特殊的地上根，其中含有固氮藻青菌。通过在地面上生长，根暴露于藻青菌生长所需的阳光下。热带土壤中自由活菌的固氮作用也是有益的。